3D micromechanisms in mushy alloy deformation

The UK metal casting industry employs ~30,000 people, contributes ~£2.5 billion per year to the economy, and plays a key role in manufacturing across the automotive, aerospace, defence, and energy sectors. However, there is strong global competition in metal casting which drives manufacturers to seek ways to minimise casting defects such as pores and cracks whilst reducing cost. This research carried out by a team of scientists from the Imperial College London, University of Manchester, and Diamond Light Source focuses on understanding the deformation behaviour of solidifying metals and the origin of casting defects so that physically-based models of defect formation can be developed.

Beamline I12 Scientific Highlight

Damaging casting defects often form late during solidification when a network of solid exists that is subjected to load. Current interpretations of semi-solid alloy deformation are based on so-called ‘post-mortem’ analyses of fully-solid samples, and many of the interpretations are mutually exclusive. Therefore, it was necessary to test the ideas developed from post-mortem studies by directly observing the phenomena in situ. Diamond’s Joint Engineering, Environment and Processing (JEEP) beamline, I12, was used to capture a series of 3D images of semi-solid deformation in real-time, enabling the team to directly test previously proposed micromechanics in semi-solid alloys and to prove the active mechanisms.

The results show that compression of the semi-solid metal leads only to compaction for a solid fraction below a certain limit. Above this limit, the compression of the slurry actually pushes the grains apart, which sucks in the melt phase and leads to the drying out of the outer surface.

A wide range of materials deform as a granular fluid, from cornstarch slurries to soils, rock, and magma flows. This work has shown that semisolid metallic alloys exhibit behaviour similar to that of mainstream granular solid-liquid mixtures, which has enabled a better understanding of alloy flow behaviour and the formation of defects during metal casting. Using high-speed synchrotron tomography on the JEEP beamline (I12) at Diamond, the discrete aluminium grain response was directly imaged and measured during uniaxial compression of aluminium-copper alloys. This showed that the stress-strain response at 64-93% solid was due to the shear-induced dilation of discrete rearranging grains. Uniaxial compression also opened pre-existing internal voids as well as drew the free surface of the specimen into the liquid, resulting in cracking under compression.

Creating an engineering component from metal involves solidifying liquid metal at some stage during processing. Solidification invariably creates casting defects such as porosity or tears, which usually develop once solidification has produced a solid network and permeability in the solidifying casting has started to drop. Understanding casting defects therefore requires an understanding of the deformation behaviour of mushy alloys with a high solid fraction.

There are currently several hypothesised micromechanisms for the deformation behaviour of semi-solid alloys with a large amount of solid, such as the viscoplastic deformation of a continuous porous solid metallic skeleton1 (similar to a liquid saturated sponge), the deagglomaeration of a flocculated metallic suspension2 (similar to dispersed clay slurries), or the granular rearrangement of quasi-rigid cohesionless metallic grains3 (similar to a saturated particulate soil). However, most of the current interpretations of mushy alloy rheology are based on bulk mechanical data and postmortem microscopy and microstructural analysis, and many of the proposed micromechanisms have never been observed in the context of large 3D specimen deformation under the right conditions. In addition, many industrial casting processes apply pressure to control porosity to varying degrees of success, with pressure being effective at reducing casting porosity4 but, in excess, leading to cracks and tears5. The link between pressure and porosity was therefore unclear.

High-speed synchrotron tomographic imaging was performed on uniaxial compression of Al-Cu alloys with globular (a-Al) grains at 64, 73, 86 and 93% solid. These experiments enabled the direct identification of the 3D grain-scale mechanisms of deformation and an understanding of how they relate to the stress-strain response and lead to casting defects over a range of solid fractions.

3D grain scale mechanisms The behaviour of individual grains during compression was studied in a manner similar to the study of the kinematics of sand grains imaged during uniaxial compression. This research has shown that each grain displaces independently, with quasi-rigid grains pushing each other apart as they translate and rotate independently under load (see example at 73% solid in Fig 1.). The stress-strain response at 64-93% solid was therefore due to the shear-induced dilation of discrete rearranging grains, where the globular semisolid metal microstructures deformed as near-cohesionless granular materials despite containing tightly packed assemblies of soft partially-cohesive grains. The resulting drawing-in of the free surface of the specimen manifesting itself as large menisci, and in the case of >73% high solid fraction, propagating into the specimen, is then an emergent phenomenon simply caused by the rearrangement of initially tightly packed quasi-rigid grains, and would not be expected if strain was only accommodated by viscoplastic deformation of the solid phase.

3D grain scale mechanisms

The behaviour of individual grains during compression was studied in a manner similar to the study of the kinematics of sand grains imaged during uniaxial compression. This research has shown that each grain displaces independently, with quasi-rigid grains pushing each other apart as they translate and rotate independently under load (see example at 73% solid in Fig 1.). The stress-strain response at 64-93% solid was therefore due to the shear-induced dilation of discrete rearranging grains, where the globular semisolid metal microstructures deformed as near-cohesionless granular materials despite containing tightly packed assemblies of soft partially-cohesive grains. The resulting drawing-in of the free surface of the specimen manifesting itself as large menisci, and in the case of >73% high solid fraction, propagating into the specimen, is then an emergent phenomenon simply caused by the rearrangement of initially tightly packed quasi-rigid grains, and would not be expected if strain was only accommodated by viscoplastic deformation of the solid phase.

Porosity in compression

It has been shown that compression can either decrease porosity in globular semi-solid alloys or increase it depending on the solid fraction: at 64% solid, pre-existing pores closed during uniaxial compression, which is the expected behaviour. In contrast, at the higher solid fraction of 93%, pores grew during uniaxial compression rather than shrinking (Fig. 2). The research has also shown that at 64% solid, there was space for grains to rearrange with local areas of compaction, where grains moved closer to each other, driving pore closure. In contrast, at 93% solid, grains were so densely-packed that their translation and rotation under load lead to shear-induced dilation which caused the opening of pre-existing pores, inducing the creation of larger internal defects and aggravating the flaws that applying pressure intended to diminish.

This research has demonstrated that semi-solid alloys act as a granular material and exhibit shear-induced dilation during uniaxial compression, which can be at the origin of large internal and external defects.

Funding acknowledgements: This work was carried out with the support of Diamond Light Source during beamtime EE6893-1 on the JEEP (I12) beamline. This work was made possible thanks to the financial support of Norsk Hydro ASA and the facilities and support provided by the Manchester X-ray Imaging Facility and the Research Complex at Harwell, funded in part by the EPSRC (EP/I02249X/1).